1. Field of the Invention
This invention relates to power converters and more particularly, to switching power converters such as flyback converters.
Conventional power flyback converters draw current from the power source into the primary winding of the transformer. The primary winding current starts at zero and ramps up with a rising edge and is then interrupted when a power switch turns off. The primary winding current then remains zero for an interval. At the point where the primary input current is interrupted, stored energy in the transformer winding causes current to flow in the secondary or output winding of the transformer. In most conventional flyback converters, a rectifier diode is provided on the secondary side of the transformer. The voltage drop across the rectifier diode affects the efficiency of the converter.
Power converters employing transistor power switching devices on the secondary side of the transformer are known such as the high frequency switching circuit disclosed in U.S. Pat. No. 5,594,629 to Steigerwald. The Steigerwald power converter includes a primary side power switching device Q1 and a secondary side power switching device Q2, which are controlled to operate in a natural zero-voltage switching mode such that the power switching devices are switched with zero-voltage across them at the time of switching. The zero-voltage switching capability permits the converter to operate with greater efficiency. The Steigerwald patent fails to disclose the control circuits or operations for causing the switching of the power switching devices Q1 and Q2.
The bi-directional DC to DC power converter disclosed in U.S. Pat. No. 6,069,804 to Ingman et al. includes an output bi-directional switch such as a FET 34. The converter increases efficiency by use of a resonant transition control means for sensing the inductor input and output winding currents and the output voltage and for adjusting the frequency to provide switching of the power switches in a resonant transition mode and to adjust the output voltage to a predetermined level. In the resonant transition mode of operation, the period of the clock circuit is adjusted to provide substantially resonant transitions on both the input and output bi-directional switches. The embodiments of the bi-directional power converter disclosed in the Ingman Patent include a control scheme for the power bi-directional switches in which the primary-side switch and the secondary-side switch are controlled by the same clock circuit 44. The second control signal on signal line 48 controlling the secondary side bi-directional switch has a state that is the compliment of the state of the first control signal on signal line 46 controlling the primary side bi-directional switch. Thus, the control signals for the primary and secondary bi-directional switches are related signals coming from the same control unit. FIG. 9 of the Ingman Patent further illustrates the clock circuit 44 acting as the control unit. More particularly, a transformer 125 drives both the primary and secondary side by directional switches.
It is an object of the invention to provide a power converter which allows zero-voltage switching on both primary and secondary side switches.
A further object of the invention is to provide a power converter of the variable frequency flyback type with improved efficiency.
It is a further object of the invention to provide a power converter which achieves full zero-voltage switching on primary and secondary side switches combined with synchronous rectification.
It is still a further object of the invention to provide a power converter having primary and secondary side switches which are controlled by separate stand-alone control units.
It is another object of the invention to provide a power converter having primary and secondary side switches which does not have control signals over the isolation barrier and which does not include a secondary transformer which must meet regulatory safety tolerances.
Additionally, it is an object of the invention to provide a power converter having primary and secondary side switches where the secondary side switch is operated as a slave.
It is still another object of the invention to provide a power converter having primary and secondary side switches of the master-slave type where the secondary side switch is controlled following the waveform of the transformer of the power converter.
It is yet another object of the invention to provide a power converter where the transformer of the power converter is allowed to be charged in a reverse direction to a particular level and then switched to be charged in the forward direction.
It is still a further object of the invention to provide a power converter having primary and secondary side switches where the secondary side switch is turned off at a predetermind level of back current.
These and other objects of the invention are accomplished by providing a power converter comprising: a transformer having a primary winding and a secondary winding, a primary side switch, a secondary side switch, a master control unit controlling the switching of the primary side switch, a slave control unit for controlling the switching of the secondary side switch in accordance with a detected backflow current in the secondary side switch.
These objects are further accomplished by providing a power converter comprising: a transformer having a primary side and a secondary side, a primary side power switch, a secondary side power switch, a secondary control unit for controlling the switching of the secondary side power switch which follows the waveform of the transformer. In this preferred embodiment of the power converter, there is also provided a primary waveform sensor and a secondary waveform sensor. The primary waveform sensor is preferably a sense winding on the primary side of the transformer, and the secondary waveform sensor is preferably a sense winding on the secondary side of the transformer.
Also disclosed is a method of power conversion comprising: regulating output power by varying the duty cycle of a primary power switch; switching a secondary side power switch in accordance with the waveform of a transformer connected between the primary and secondary side power switches.
The above and other objects, aspects, features and advantages of the invention will be more readily apparent from the description of the preferred embodiments thereof taken in conjunction with the accompanying drawings and appended claims.